Gauging Locks.
When canals were constructed it was common practise for the near end locks to be built as gauging locks with restricted width. On the Worcs Bham the Blockhouse Lock (relic of the Civil War)2 up from Diglis, is 7 ft exactly which catches out some old , and new , boats. I remember in the 1960's seeing the young crew of a wooden conversion stuck fast. They went shopping in the City and returned with bars of Sunlight Soap After an hours vigorous rubbing they squeezed through in a cloud of bubbles.Happy Days. Is Sunlight still available, Fairy Liquid wouldn't work.